1. Technical Field
The present disclosure relates to using electromagnetic energy to treat tissue. More particularly, the present disclosure relates to a system and method of matching impedances of an electrosurgical generator and/or a microwave generator.
2. Description of Related Art
Historically, surgery was performed using only mechanical tools, such as mechanical cutting instruments, scalpels, bladed forceps, saws and the like. However, in recent years, technology has improved such that surgeons now frequently use electromagnetic waves to cause a wider variety of surgical effects, e.g., by selectively modifying tissue using electromagnetic energy to produce a specific effect. The characteristics of the electromagnetic energy applied to tissue strongly correlates to the effect that the energy has on the tissue. These characteristics are therefore changed in accordance with the desired tissue effect. Two types of electromagnetic energy that are commonly applied during surgery include electrosurgical energy and microwave energy.
Electrosurgery is surgery using electrosurgical energy. Electrosurgery involves the application of electrosurgical energy (e.g., “RF energy”) to cut, dissect, ablate, coagulate, seal tissue, or otherwise treat biological tissue during a surgical procedure. Additionally, certain electrosurgical modes invoke the application of electric spark to biological tissue, for example, human flesh or the tissue of internal organs, without significant cutting. The spark is produced by bursts of radio-frequency electrical energy generated from an appropriate electrosurgical generator. Generally, fulguration is used to coagulate, cut or blend body tissue. Coagulation is defined as a process of desiccating tissue wherein the tissue cells are ruptured and dehydrated/dried. Electrosurgical cutting, on the other hand, includes applying an electrical spark to tissue in order to produce a cutting or dividing effect. Blending includes the function of cutting combined with the production of a hemostasis effect.
Generally, electrosurgery utilizes an electrosurgical generator, an active electrode and a return electrode. The electrosurgical generator generates electrosurgical energy typically above 100 kilohertz to avoid muscle and/or nerve stimulation between the active and return electrodes when applied to tissue. During electrosurgery, current generated by the electrosurgical generator is conducted through the patient's tissue disposed between the two electrodes. The electrosurgical energy is returned to the electrosurgical source via a return electrode pad positioned under a patient (i.e., a monopolar system configuration) or a smaller return electrode positionable in bodily contact with or immediately adjacent to the surgical site (i.e., a bipolar system configuration). The current causes the tissue to heat up as the electromagnetic wave overcomes the tissue's impedance. Although many other variables affect the total heating of the tissue, usually more current density directly correlates to increased heating.
Microwave surgical procedures invoke the application of microwave energy to tissue. Unlike low frequency RF therapy that heats tissue with current, microwave therapy heats tissue within the electromagnetic field delivered by an energy delivery device (e.g., a microwave antenna). Microwave surgical procedures typically utilize a microwave generator and an energy delivery device that delivers the microwave energy to the target tissue. One type of energy delivery device is a coaxial microwave antenna that forms an approximate dipole antenna. Microwave surgical systems involve applying microwave radiation to heat, ablate and/or coagulate tissue. For example, treatment of certain diseases requires destruction of malignant tissue growths (e.g., tumors) or surrounding tissue. It is known that tumor cells denature at elevated temperatures that are slightly lower than temperatures injurious to surrounding healthy cells. Therefore, by applying microwave energy to heat tumor cells to temperatures above 41° C. kills the tumor cells while adjacent healthy cells are maintained at lower temperatures avoiding irreversible cell damage. Another method used to treat diseased tissue is to resect a portion of the diseased organ, tissue or anatomical structure. For example, a liver may contain diseased tissue and healthy tissue. One treatment option is to pre-coagulate and ablate some of the liver tissue to facilitate resection of a portion of the liver including the diseased tissue. Microwave energy can be used during these types of procedures to pre-coagulate tissue prior to resection, to reduce bleeding during resection and to facilitate the actual resection of the tissue.
The microwave energy may be applied via an antenna that can penetrate tissue. There are several types of microwave antennas, such as monopole and dipole antennas. In monopole and dipole antennas, most of the microwave energy radiates perpendicularly away from the axis of the conductor. A monopole antenna includes a single, elongated conductor that transmits the microwave energy. A typical dipole antenna has two elongated conductors parallel to each other and positioned end-to-end relative to one another with an insulator placed therebetween. Each of the conductors is typically about ¼ of the length of the wavelength of the microwave energy making the aggregate length of both conductors about ½ of the wavelength of the microwave energy.
Both sources of energy used during modern surgical procedures share some common characteristics. Microwave and electrosurgical generators both apply electromagnetic energy and include input impedances. Also, each utilizes a surgical instrument that directs the energy to the target tissue. Additionally, while the energy is applied to the tissue, the tissue is affected and its characteristics change as a result of the application of the energy. For example, microwave energy can cause tissue to heat up causing water to boil therein. This reduction in hydration causes the tissue's impedance to increase. Likewise, an electrosurgical instrument can also cause tissue to produce an analogous increase in impedance from dehydration when applying electrosurgical energy thereto.